C-13 NMR and crystallographic evidence of push-pull effects in furoindolicbeta-enamino esters

A series of cyclic enamine derivatives, stabilized through conjugation with an electron-attracting CO2Me group at the beta-carbon of the enamine syste m, has been prepared. The influence of substituents at the vinylogous donor NH2 group on C-13 chemical shifts has been investigated, and the results a nalyzed on the basis of resonance and steric effects on the dipolar charge distribution in the molecule. Intramolecular H-bonding was established for the N-methylated enamines 4a and 4b based on the high-frequency shift of th e NH proton and its solvent immutability. Several deuterium-induced isotope effects on C-13 chemical shift [(n)Delta C(ND)] due to deuteration at the enamine nitrogen have been measured. The crystal structures of the enamine 1b and the N,N-diacetylated enamine 3a, determined by X-ray analyses, show that these compounds, in the solid state, adopt an s-trans disposition arou nd the C--C=O bond, with the carbonyl ester group and the C=C double bond i n an almost coplanar conformation. Comparison of bond lengths of 1b and 3a in the crystalline state provides information on changes in the extent of n -pi delocalization arising from the nature of the N-substituent.